1. Field of the Invention
The present invention generally relates to the field of micro-electro-mechanical (MEM) sensors for measuring an applied force, and more specifically to a tunneling rate based gyro.
2. Description of the Related Art
One method for sensing physical quantities such as linear acceleration or rotation, or acoustic or hydrophonic pressure is to provide a flexible member that flexes in response to an applied force and measures the amount of flex electrically. Conventional micro-mechanical techniques for achieving the transduction include capacitive coupling, piezoresistive and piezoelectric sensing. However, none of these techniques are inherently as sensitive as tunneling tip transduction.
In tunneling tip sensors, a bias voltage is applied across a flexible counter electrode and a tunneling tip with a sufficiently small gap between the two components to induce a tunneling current to flow. The tunneling current I.sub.T is given by: I.sub.T .varies.V.sub.B exp(-.alpha.h.sqroot..PHI.), where V.sub.B is the bias voltage, .alpha. is a constant, h is the electrode-to-tip separation and .PHI. is the work function. As the applied force changes, the separation between the electrode and the tip changes and modulates the tunneling current, which varies by approximately a factor of two for each angstrom (.ANG.) of electrode deflection. Tunneling currents are typically 1nA and the current noise is typically 10.sup.-13 A per square root hertz. Thus, tunneling tip detectors can provide a much greater sensitivity and a larger bandwidth than previous method of detections and still provide easily measurable signals.
For the specific application of a tunneling tip sensor as an accelerometer, the deflection distance x=ma/k, where m is the electrode's mass, k is the electrode's spring constant and a is the acceleration. The effective bandwidth of the accelerometer is determined by its resonant frequency ##EQU1## Since tunneling tip techniques are more sensitive to deflection, the accelerometer's mass can be relatively small, and thus its bandwidth can be larger than the capacitive coupling and piezoresistive devices.
A single cantilever beam and a dual-tine cantilever rotational sensor and fabrication method are disclosed in a co-pending application by similar inventors, and entitled "Single Wafer Tunneling Sensor and Low Cost IC Manufacturing Method." A single cantilever beam rotational sensor includes a z-axis sensor and a control circuit. In addition, a single lateral control electrode is disposed adjacent to the cantilever electrode of the sensor. The lateral control electrode is modulated with a voltage to induce a lateral vibration at a known maximum velocity V.sub.1. The sensor measures the Coriolis force F.sub.c, given by: F.sub.c =1/2*m*W.sub.r .times.V.sub.I where m is the cantilever electrode's mass, W.sub.r is the rotational rate and V.sub.I is the cantilever electrode's lateral velocity. The rotational rate can be determined by measuring the Coriolis force, which is directly proportional to the rotation. Although this single cantilever beam gyro provides a more sensitive and compact sensor than the other conventional rotational sensors, its structure has deficiencies. For example, linear accelerations can produce additional deflections of the cantilever electrode, causing fluctuations is the tunneling current and therefore incorrect estimates of the rotational rate. In addition, constant tunneling current cannot be guaranteed with a single cantilever beam located over a tunneling tip electrode. For large lateral excursions of the cantilever, the tunneling current between the cantilever electrode and the tunneling tip would be lost.
In addition to the single beam, surface micromachined, z-axis rotational sensor, the co-pending application discloses a rotational sensor which is insensitive to linear acceleration forces. A double-ended tuning fork is suspended above and parallel to a substrate by a cross-beam which is supported at its ends by two posts and is orthogonal to the tuning fork. One end of the tuning fork forks into a pair of cantilever beams which are positioned parallel to a rotation axis and have associated lateral control electrodes. The other end of the tuning fork forks into a pair of cantilever electrodes which are suspended above respective control electrodes and tunneling tip electrodes. The tunneling tip electrodes are connected to control circuits to maintain constant tunneling currents. The forked ends are interconnected by a cantilever member which is attached to the cross-beam.
The voltages applied to the lateral electrodes are modulated in synchronism such that their cantilever beams move 180.degree. out of phase with one another in the plane of the substrate. As the sensor rotates around its axis, equal but opposite z-axis Coriolis forces are applied to the respective cantilever beams to move them perpendicular to the surface of the wafer, producing a torque on the cantilever member proportional to the Coriolis force. The torque tries to deflect the cantilever electrodes but is opposed by the feedback circuitry. The rotational rate can be determined by taking the difference between the respective outputs. By oscillating the cantilever beams 180.degree. out of phase with one another, the changes in the positions of both cantilevers due to linear accelerations are subtracted out and do not affect the rotation signal. Although this double-ended tuning fork configuration improves performance by separating the sensor fork from the drive fork thereby reducing the noise in the rotation signal, its fabrication method and structure have several deficiencies. A robust design requires that the dual double-ended tuning fork be firmly attached to the cross-beam by adding capping material at the junction, thus requiring additional processing steps. The complex device geometry leads to the generation of many eigenmodes near the drive and sense frequencies. This reduces the effective bandwidth of the gyro device. For the double-ended tuning fork devices, additional control loops are required to electronically damp these oscillations if they are excited. These additional eigenmodes can urge the cantilever electrode beyond the tunneling electrode so that the tunneling signal is lost part of the time.
One advantage of the double cantilever design is that by driving the two tines out of phase 180.degree., linear accelerations can be rejected. However, these linear accelerations must occur at precisely the same frequency and phase as the lateral control electrode drive signals to the gyro in order to interfere with the rotational signals. By designing simple mechanical structures whose fundamental modes have frequencies far beyond that which occurs in the sensor's environment, these linear accelerations will not create error signals.